The objectives of this project are: (a) to isolate the features of speech and other auditory signals that are reinforcing at birth and to determine whether the reinforcing properties change with age; (b) to determine what features of speech very young infants perceive and whether and how the perception of speech changes in the first few months of life; (c) to determine the characteristics and chart the development of vocalization across the first year of life; (d) to relate the characteristics of infants' vocalization to their perceptual abilities; and (e) to trace the development of peer interactions across the first year of life.